Guessed by @TruckOSaurus
When Metroid tries to go for linearity and story over freedom and isolation, it can quite infamously go very wrong, and I suppose you could say it did for this game too, considering it is the worst game in the Prime Trilogy, but this is still one hell of a ride. Metroid Prime 3 has a pretty boring story that's somewhat out of character with the rest of the series, and is made worse by having bad voice acting and being forced into the player as one of the main points of the game - it's also the only mainline Metroid game where you actually do visit several planets, and while the idea of space travel is fun, the novelty quickly wears off as it doesn't change anything about the gameplay of the series, instead only separating individual areas further off in an artificial way, and making each area more linear to traverse, as well as making each planet itself less interesting than it could've been. The game starts with an explosive prologue that's not quite as fun as it is exciting, and from then on it only gets worse (a no-no for any decent Metroid game, really), as the planets you explore progressively become shallower and least interesting - Bryyo, the first of them, being by far the most fleshed out place in the game, with plenty of cool lore, different locales that all look great, and all in all just making me wish the whole game took place on it. Still, I'm glad that the idea of a Metroid game with several planets has been done, because otherwise we'd always end up wishing it'd happen - now we can get back to the stuff that makes the series good.